In machinery which is automated, for example, machine tools, printers etc, it is well known to use line grating pairs as position indicating devices. Conventionally, the gratings are moved relative to each other while a light is shone through the gratings which creates an alternate light/dark bar pattern. The bar pattern is sensed by a detector such as photocell placed on the opposite side of the gratings. Signals generated by the photocell then become a measure of the distance travelled by the grating, and if integrated over time also give a measure of the velocity of the relative motion between the grating pairs.
Such a conventional system has certain disadvantages. For example, it is difficult to position the gratings so that the lines of each grating are aligned properly in parallel relationship, and the grating surfaces themselves must be mounted close enough together to produce a sharp pattern. If there is any misalignment between the two gratings, the light/dark ratio may be less than the optimum output generated by the detector. The result of such misalignment or optical distortion results in missed signals which creates problems with regard to the machinery being controlled by the detector output. The tolerances between two moving line gratings must be held despite changes in atmospheric conditions, the normal wear of moving parts, and the all to real possibility of the gratings moving together and touching one another creating scratches or obliteration of one or more of the lines of the grating. Moreover, the risk of the gratings touching the optical parts creating damage on for example either the detector or the lens further creates a risk of improper generation of the dark and light patterns resulting in malfunctions and creating the possibility of replacing the rather expensive gratings.
In U.S. Pat. No. 3,524,067, issued on Aug. 11, 1970, a compact line grating position sensing device is disclosed, the device comprising a movable line grating with a source of illumination and a detector both mounted on the same side of the grating with means for deflecting and focusing the image of the grating produced by the illumination of the grating back on itself at a point in front of the detector. When the single grating is moved sideways relative to the light source and detector, in a direction perpendicular to the lines of the grating, interference between the grating and the image lines produces alternate illumination and non-illumination of the detector.
Similarly, in Japanese published patent application No. 11793/61, especially FIG. 3 thereof, an optical position sensing scheme is illustrated which includes a grating, and a light source and detector on the same side of the grating. An optical system which includes lenses and a prism reflects and focuses the grating line image produced by the source back upon the grating at a position opposite the detector. As disclosed therein, when the grating is moved relative to the source, the optical system and the detector and the grating line image produced by the source is superimposed on the grating at a position opposite the detector. Accordingly, interference occurs between the grating lines and the image lines to produce alternate illumination and non-illumination images to the detector. In this scheme, the lenses are apparently at a distance of twice the focal length because they are disposed so as to focus the grating line image onto the grating, while the prism is placed behind the lenses at a distance equal to the focal length. In U.S. Pat. No. 3,524,067 and Japanese published patent application No. 11793/61, the apparatus appears to be only capable of position sensing.
In U.S. Pat. No. 3,496,364 issued on Feb. 17, 1970, is disclosed a linear encoder having a fringe pattern produced by optical imaging. In this system, described in the aforementioned patent, a single ruled scale is illustrated utilizing an optical system which superimposes a rotated image of one portion of the scale upon a second portion of the scale, the image being rotated 180.degree. in the plane of the scale. Movement of the scale in one direction causes the image of the scale to move in the opposite direction. This results in light passing through the second portion of the scale being modulated by the relative movement between the scale rulings and the image of the rulings. By placing two or more photosensitive detectors at the second portion of the scale, the detectors being separated along a length of the scale by a distance suitable to produce out-of-phase electrical signals in response to light passing through the second portion of the scale, the sense of the phase of these signals is a representation of the direction of scale motion while the number of cycles of the signal is representative of the amount of scale motion. In this latter system, the phase information is obtained by generating a moire fringe pattern so that the pattern movement is at a speed or direction or both different than the movement of the encoder scale. This means, however, that some magnification of the image is necessary.
In view of the above, it is a principle object of the present invention to provide novel yet simple apparatus which will permit of accurate position detection between an assembly and a grating having relative motion therebetween, as well as the direction of relative motion.
Another object of the present invention is to provide a self-imaging grating detection apparatus in which the position of the light source and photodetector is insensitive to both grating rotation and the positioning of the detector and light source assemblies.
Still another object of the present invention is to provide a bidirectional self-imaging grating detection apparatus in which no comparison grating is necessary and therefore the grating resolution may be changed simply by changing the grating strip.
Other objects and a more complete understanding of the invention may be had by referring to the following specification and claims taken in conjunction with the accompanying drawings in which: